This invention relates to improvements in an assembly employing a hollow Luer nut to draw the ends of two hollow tubes together so as to form a continuous passageway therethrough. In this assembly, the outer end of a male tube becomes smaller towards an end thereof so as to form a conical surface that can be seated in an axial conical recess in an end of the female tube so as to form a sealed joint. An outer annular ridge or shoulder is formed adjacent to the larger end of the outer conical surface of the male tube, and an outer annular projection is formed about the end of the female tube. The Luer nut has an annular collar at one end and internal threads in the other, and does not deform during use. It is positioned with the male tube passing through the collar, the annular ridge on the male tube inside the collar, and the annular projection on the female tube engaging the threads. As the nut is screwed onto the annular projection, the collar contracts the ridge on the male tube and moves it toward the female tube until the conical end of the male tube is seated in the conical recess of the female tube so as to form a seal.
The normal Luer nut has a 10-pitch thread with 2 starts. Less than one turn of the nut is required to couple or decouple the tubes. Unfortunately, however, the nut is often loosened during normal handling or just relaxing and the seal broken. This is due to the high angular contact of the threads. The thread contact point is a steep wedge that is likely to unloosen because of the steep angle of contact. Some improvement was attempted by reducing the diameter of the threads as the interior of the nut was approached so as to increase the frictional torque as the nut was screwed into position. In some assemblies, however, the dimension of the threads in the nut and of the annular projection at the end of the female tube, although within normal manufacturing tolerances, were such that the frictional torque become too great for an operator to easily overcome before the male tube seated firmly enough in the female tube to form a seal. This is prevalent in a round female flange and is caused by the round shape of the nut being extended radially outward until the nut resists further radial expansion because the walls of the nut are in tension or hoop stress.
In assembling a tubing system with Luer nuts it is often desirable or even necessary to be able to rotate one tube with respect to the other without loosening the seal between the tubes. Such a situation may be encountered where one or both tubes being sealed is stiff, as, for example, when one or both are part of a molded valve. Not having an adequate seal can be life threatening, and an assembly in which this condition can be brought about without the technician being aware of it is therefore particularly dangerous.